The present disclosure relates to voltage regulators.
Electronic circuits typically operate using a constant supply voltage. A voltage regulator is a circuit that can provide a constant supply voltage, and includes circuitry that continuously maintains an output of the voltage regulator, i.e., the supply voltage, at a predetermined value regardless of changes in load current or input voltage to the voltage regulator. For example, a battery used to power a mobile device may have a decreasing output voltage as the battery loses charge. A voltage regulator can supply a constant voltage to a load as long as the output voltage of the battery is greater than the constant voltage supplied to the load. The load can be any type of electronic circuit that receives a substantially constant voltage source. For example, the load may be a processor in a mobile device that has integrated functions such as wireless communication, image capture, and a user interface. Since tasks of the processor vary according to usage of the mobile device, the load the regulator must respond to are always changing.
One type of voltage regulator is a low-dropout regulator (LDO). A LDO is a DC linear voltage regulator that can regulate a supply voltage even when the input voltage to the LDO is very close to the supply voltage. The drop-out voltage of a voltage regulator is the minimum voltage difference that must be present from an input of the regulator to an output of the regulator for the regulator to provide a constant supply voltage. LDOs are voltage regulators that have a low drop-out voltage, e.g., lower than 50 mV.
FIG. 1 shows a conventional LDO 100 that provides a regulated output voltage VOUT from a power source voltage VPOWER provided by a power supply, such as a battery, a transformer, or other voltage source (not shown). A fraction of the output voltage is fed back to an inverting input of an amplifier, e.g., a differential amplifier 102, through a resistor divider network including resistors R1 and R2, which makes the LDO 100 function in a closed loop. The feedback voltage VFB is compared with a reference voltage VREF provided to a non-inverting input of the amplifier 102. The output of the amplifier 102 is a voltage that is modulated as a function of the difference between the feedback voltage VFB and the reference voltage VREF. The amplifier 102 provides the modulated voltage to the gate terminal of a pass element, e.g., pass transistor MN. The amplifier 102 controls the current through the pass transistor MN to control the output voltage VOUT. Hence, a steady voltage is attained at VOUT. In steady state, the voltage VOUT is regulated around its nominal value which is equal to [(R2+R1) VREF/R1].
While FIG. 1 includes the pass transistor MN as the pass element, any suitable pass element can be used. Examples of pass elements include Darlington circuits, NMOS (n-channel Metal Oxide Semiconductor) and PMOS (p-channel Metal Oxide Semiconductor) transistors, and NPN and PNP bipolar transistors. When a p-channel transistor, e.g., a PMOS transistor, is used as the pass element, the feedback voltage VFB is provided to the non-inverting input of the amplifier 102 and the reference voltage VREF is provided to the inverting input of the amplifier 102.
The transfer function of the LDO 100 has three poles and one zero. The dominant pole is set by the amplifier 102, and is controlled and fixed in conjunction with the transconductance gm of the amplifier 102. The second pole is set by the output elements, namely, the combination of the output capacitance of capacitor COUT and the load capacitance and resistance. The third pole is due to parasitic capacitance around the pass transistor MN. Because the load current ILOAD can vary between 1 μA to 100 mA, the second pole of the LDO 100, being affected by the load capacitance and resistance, can vary greatly, resulting in a feedback loop that can be difficult to stabilize for all load conditions.